The invention relates to a shield arrangement for a vacuum cell.
Total vacuum cells are utilized to measure the total pressure in vacuum chambers. The vacuum measuring cells are employed when monitoring various vacuum processes. Typical vacuum processes are here surface treatment processes, such as for example coating methods and etching methods. Such methods operate for example in the pressure ranges of 10−5 mbar up to 100 mbar. It is conventional to measure such pressure ranges using so-called Pirani vacuum measuring cells or with so-called diaphragm vacuum measuring cells. One problem encountered herein is that the process gas, but also residual gas components, may contaminate the vacuum sensor during the process. Consequently, imprecise or error measurements or pressure indications, may herein result. The vacuum measuring cells exhibit a drift behavior, corresponding to the time during which they are exposed to the process, which cannot always be eliminated or restored by cleaning the vacuum cell or its surrounding. Diaphragm measuring cells are especially sensitive with respect to possible contaminations. In such diaphragm measuring cells a thin diaphragm is deflected depending on the obtaining pressure which is to be measured. The deflection of this diaphragm is measured and serves as a measure for the vacuum pressure to be measured. In the case of capacitive diaphragm measuring cells the deflection of the diaphragm is measured via the change of capacitance between the diaphragm and the solid body. In the case of optical diaphragm measuring cells this deflection is acquired with optical methods, for example with methods of interferometry. In order to be able to measure such pressure ranges with high sensitivity, the diaphragms must be implemented very thin, for example in the range of 50 to 760 μm. Contaminations of this thin diaphragm, for example with gas and/or with particles, can lead to tensile and/or pressure stress, which additionally affects the deformation of the diaphragm and subsequently leads to error measurements, for example in the absolute values to be measured, or to undesirable drift behavior overtime. In addition, herein the resolution, and consequently the precision, of the measuring cell is decreased and, on the other hand, the reproducibility of the measurement results is not ensured. In order to decrease such contaminations a flat shield, also referred to as baffle by experts in the field, has previously been utilized, as is shown by the example of a capacitive diaphragm vacuum cell in FIG. 1. The vacuum cell 15 is comprised of a first flat and round housing part 1 and a second flat and round housing part 4, between these two housing parts a diaphragm 2 is connected via seals 3, for example a glass solder, forming a sealing such that between the diaphragm and the two housing parts one cavity 9, 10 each is implemented. The one cavity forms a reference vacuum space 10 which communicates across a connection 13 with a getter space 12. In the getter space 12 is disposed a getter 11 for the reliable maintenance of a reference vacuum. Opposite the reference vacuum space 10 on the other side of diaphragm 2 is formed the measuring vacuum space 9, which communicates across an exit port 16 with the shield housing 6, in which a shield 7 is disposed, the shield housing being appropriately connected with the vacuum cell 15 for example via a connection fitting 5. On the shield housing 6 is disposed a connection flange 8 with a connection opening 22, which can be connected with the vacuum process chamber to be measured. The connection opening 22 is so disposed that the shield 7 opposite the exit port 16 of the vacuum cell does not permit a direct through-view: the shield 7 is to deploy its protective effect thereby that the undesirable gases or particles are condensed on the shield surface such that they do not reach into the vacuum cell. In the technical literature this shield is often also referred to as plasma shield. In processes containing reactive gases, these are intended to be preferably condensed on the shield. The sensor drift is hereby intended to be decreased and thereby the service life of the measuring cell to be increased. Although this flat shield improves the service life of the measuring cell, it is not possible to prevent that there is still an appreciable fraction of particles, for example also facilitated through scatter processes, about the shield reach the measuring diaphragm and here falsify the measurement.